The present invention relates generally to medical equipment, and more particularly, to a closed loop control system for a high frequency oscillating ventilator (HFOV) for producing positive and negative pressure waves in respiratory air that is supplied to a patient. Advantageously, the control system is specifically adapted to allow for accurate control of the pressure oscillations produced by the HFOV while providing a means for accurate control of mean airway pressure (MAP).
As opposed to conventional ventilators which ventilate only during the inhalation phase and which rely on human physiological response for ventilation during the expiration phase, HFOV's produce an active exhalation which is critical in the respiration of certain types of patients such as in neonates and/or other child or adult patients suffering from certain lung diseases. In some cases, the lungs of the patient may be incapable of providing adequate ventilation or gas exchange, particularly during the exhalation phase. In this regard, HFOV's are specifically developed to provide sufficient gas exchange and full oxygenation of a patient whose respiratory abilities in the exhalation phase are compromised.
In a simplified description of an HFOV patient ventilation system, HFOV's typically deliver a relatively small tidal volume to the patient while simultaneously keeping the lungs and alveoli open at a relatively constant airway pressure. The small tidal volume is delivered to the lungs at a relatively fast rate typically measured in breaths per second or Hertz (Hz) wherein 1 Hz is equal to 1 breath per second. HFOV's typically operate at respiratory rates that greatly exceed the normal breathing rate of a human. For example, HFOV's may operate at a rate of 5 Hz (i.e., 5 breaths per second) whereas the at-rest breathing rate of an adult human is typically less than 1 Hz.
The positive and negative pressure waves or pressure oscillations that are delivered to the patient are typically generated by a piston disposed within the HFOV. The piston is adapted to rapidly move an elastic diaphragm at the desired frequency. The piston may be driven by a linear motor powered by a square-wave driver which induces rapid reciprocation caused by switching of the polarity of the square-wave driver between positive and negative values. Variations in the polarity voltage or current at the square-wave driver cause proportional increases or decreases in piston amplitude.
Because the piston displacement causes the pressure oscillations, the greater the piston amplitude, the greater the tidal volume delivered to the patient. Certain patients may have high resistance in the airway which, in turn, generates greater resistance against which the piston must act during its reciprocation. Therefore, accurate control of the piston movement (e.g., amplitude) is desirable in order to provide the optimal amount of ventilation to the patient.
Furthermore, because HFOV's must also keep the lungs and alveoli open at a generally constant airway pressure and because different patients have differing levels of airway resistance, it is further desirable that MAP is adjustable. Regulation of the MAP in the patient ventilation system is typically facilitated by means of an exhalation valve disposed adjacent to a patient wye. Ideally, the MAP is adjusted such that the lungs and alveoli are maintained at an open state in order to prevent lung inflate/deflate cycles which may be damaging to alveoli over time and which may lead to further complications.
The prior art includes a wide variety of HFOV's that are directed towards generating pressure pulses in the patient circuit or lung system. For example, U.S. Pat. No. 4,409,977 issued to Bisera et al. and entitled High Frequency Ventilator discloses a high frequency breathing apparatus adapted to deliver high frequency air pulses to a catheter that is energized by pressurized air. The Bisera device includes a flexible bag forming a sealed chamber and a pressure source that provides pressure pulses to the chamber to compress the bag at each pulse and thereby deliver air into a catheter that leads to a patient's lungs.
U.S. Pat. No. 4,719,910 issued to Jensen and entitled Oscillating Ventilator and Method discloses an HFOV having a diaphragmatically sealed piston mounted in a housing and being reciprocative therewithin via circuitry that is operable to alternatingly reverse the polarity of current flow into a motor. The Jensen device is connected to the patient's airway via a tube to deliver a polarized pressure wave within a flow of gas delivered to the patient.
U.S. Pat. No. 4,788,974 issued to Phuc and entitled High Frequency Artificial Respirator discloses a high frequency respirator wherein a patient's circuit thereof is supplied with respiration gas and an oscillation generator imparts high frequency oscillation to the respiration gas. Pressure waves created thereby are delivered to gas flow within the patient's circuit which aids diffusion of gas within the air passages and giving artificial respiration to the patient.
U.S. Pat. No. 5,704,346 issued to Inoue and entitled High Frequency Oscillatory Ventilator discloses a device for transmitting high frequency pressure to a patient via reciprocation of a piston. The Inoue device utilizes a soft bag for absorbing pressure generated by the piston and which allows for adjusting the amount of gas exchange in the patient without altering piston stroke such that a rotary motor may be used for driving the piston.
U.S. Pat. No. 6,640,807 issued to Bennarsten and entitled High Frequency Oscillation Ventilator discloses an HFOV for alternately supplying and removing a volume of gas to and from a patient. The Bennarsten HFOV includes a flow controller for apportioning the volume of gas supplied by the unit in order to establish a desired inspiration tidal volume for delivery to the patient independent of the oscillator volume.
U.S. Pat. No. 4,617,637 issued to Chu et al. and entitled Servo Control System for a Reciprocating Piston Ventilator discloses a control system for moving a piston of an HFOV utilizing non-linear time domain analysis in a predictive servo control system for controlling reciprocative movement of the piston. The predictive servo control system utilizes flow profiles stored in look-up tables as well as pressure and positional information in order to generate control signals for regulating movement of the piston.
Many of the techniques utilized in controlling the above-mentioned HFOV's employ open loop control systems wherein input signals representative of various operating parameters of the HFOV are fed to a controller in order to achieve the desired ventilation characteristics such as MAP and tidal volume. Unfortunately, due to changes in the operating parameters of the HFOV during ventilation of the patient as well as due to changes in physiological response of the patient, inaccuracies may develop in the patient ventilation system including errors in the MAP and errors in piston movement (amplitude and/or frequency). Such inaccuracies may lead to less-than-optimal efficacy of patient ventilation with the risk of complications developing in the patient.
The prior art includes several attempts to overcome the above-mentioned problems of open loop control systems for HFOV's. For example, the Chu reference appears to disclose a servo control system that employs system feedback in the form of flow rate and pressure measurements in an attempt to control MAP with greater accuracy. However, such feedback is only indirectly utilized to control piston movement. In this regard, the control system disclosed in the Chu reference is understood to be a predictive control system that uses a synthetic empirical approach rather than a systematic control system approach that directly utilizes feedback on critical parameters of patient ventilation (i.e., patient circuit pressure) and HFOV operation (i.e., piston movement).
In this regard, prior art control systems, as understood, fail to address the conflicting control goals of HFOV's. More specifically, many of the prior art HFOV's employ control systems that do not allow for accurate control of pressure oscillations produced by the HFOV nor allow for accurate centering of the reciprocating mechanism (i.e., the piston) such that piston amplitude can be maximized without regard to frequency. Without the benefit of centering controls for the piston, closed loop pressure controls alone will cause the piston to drift toward one of opposite ends of its stroke and ultimately interfere with the ability to maintain oscillations. Furthermore, many of the prior art HFOV's are not understood to provide a means for attenuating or decoupling pressure oscillations from MAP control. Finally, the control systems of the prior art HFOV's are not understood to provide an effective and accurate means for improving the rate of response to changes in the MAP.